Huixtla, Mexico - Edith Cruz was sitting at home in central Honduras, scanning Facebook on her phone, when she saw the post about the caravan on a community news page.

It was October 12. She and her cousin had just opened a small business selling tortillas when they were confronted by a gang, threatened with death if they didn't hand over half of their profits. She looked at the Facebook post: "An avalanche of Hondurans is preparing to leave in a caravan to the United States. Share this!" Within three hours, her bags were packed.

The question of how the migrant caravan began has wound its way to the American midterm elections. President Donald Trump and other Republicans have suggested that Democrats paid migrants to begin the journey. As the group continues to grow, the largest such caravan in recent years, its beginnings are being scrutinised: How did more than 5 000 migrants from across Central America find each other?

Although the caravan's origin story remains somewhat opaque, the answer from many migrants here is that they had wanted to leave for months or years, and then - in a Facebook post, a television programme, a WhatsApp group - they saw an image of the growing group and decided.

"Right away, I knew I would go," said Irma Rosales, 37, from Santa Ana, El Salvador, who saw images of the caravan on television and bought a bus ticket to meet up with the group in Guatemala last week.

"I had been waiting for a way to get north, and then I heard about the caravan," said Ediberto Fuentes, 30, who had fled Honduras for southern Mexico but was stranded for months, without the money to pay for a smuggler to travel to the United States.

"I packed my bag in 30 minutes," said Jose Mejia, 16, from Ocotepeque, Honduras, who heard about the caravan when his friend knocked on his door at 4 a.m. and said simply, "We're going."

On Tuesday, they stopped to rest in the small southern Mexican city of Huixtla, washing their clothes in buckets of water, sending messages to their families from Internet cafes, accepting whatever donations local residents were willing to offer. There was word that hundreds more migrants from across Central America, drawn by the endless media coverage, were on their way.

The Honduran government claims that community activists, led by a former legislator named Bartolo Fuentes, were initially behind the group, intending to malign the country's leaders. The bulk of the migrants here are still from Honduras.

"There's clear evidence where it began. Bartolo was the person who was in front of the media; he was the face of this event," Alden Rivera Montes, Honduras' ambassador to Mexico, said in an interview.

"They were trying to show Honduras as a failed country, which is totally false," Rivera Montes said.

Vice President Mike Pence said in an interview with The Washington Post on Tuesday that Honduras' president told him the caravan was financed by Venezuela's left-wing government. There is no evidence to support that claim.

Fuentes told The Post he was merely helping to connect small groups of would-be migrants who were already planning to travel north. In September, there were posts on Honduran Facebook groups about the plans for the caravan.

"These people who have normally migrated, hidden, day after day, had decided to come together and travel together to protect themselves," Fuentes said.

He said he was in touch with four groups of would-be migrants who were talking on WhatsApp and other social networks - in Tegucigalpa, the capital, as well as La Ceiba, Colon and San Pedro Sula - about the possibility of travelling together.

"They contacted me; they said, 'We saw what you've written; we want you to tell us how the caravan had gone in March,' " he said.

Fuentes had a long career as a political activist on the Honduran left. A former student leader who had protested against the U.S.-backed "contra" war to overthrow the neighbouring Nicaraguan government, he was elected to the legislature in 2013 and hosted a radio show about migration called "Without Borders." He is a staunch critic of President Juan Orlando Hernández.

A week before the caravan started, Fuentes posted on his Facebook page a flier about the caravan that read, "We aren't going because we want to, violence and poverty is driving us out." It called people to meet at 8 a.m. on October 12 at the San Pedro Sula bus terminal.

"We are going to accompany these people," Fuentes wrote on Facebook on October 5. "We will support them at least for the departure."

The early days of the caravan received a surge of media coverage in Honduras, particularly from HCH, a popular television broadcaster in the country. By the time people started gathering at the bus terminal on October 11 and 12, there were live streams on various Facebook pages. Before Americans had heard about it, the caravan had gone viral in Central America.

"Everyone wants to know who is guilty, who is behind this," said Irineo Mujica, director of Tijuana-based Pueblos Sin Fronteras, which has advocated for this and previous caravans, helping to arrange the routes and other logistics. "But no one has the power to organise this many people. No one can engineer an exodus."

By mid-October, the explosion of media coverage and viral social media posts across Central America prompted an explosion in the number of migrants. Within days of the caravan's departure from San Pedro Sula on October 13, almost no one could pin down the group's official origin story. They could cite only the Facebook post or television program that led to their own decision to migrate.

Many of the migrants watched the caravan grow in real time, surprised as the numbers surged.

"When I arrived at the bus terminal (in San Pedro Sula), there were 30 people. A few hours later, there were hundreds," said Jose Vijin, 32, from northwestern Honduras.

Migrant caravans have travelled through Central America for several years, part human rights protest, part effort to guarantee safe passage for Central Americans traversing a dangerous route north. Normally, a Central American migrating to the United States must pay a series of cartel-linked smugglers to make the journey, a sum that can reach more than $10 000. The caravan offered a relatively safe way to migrate that was basically free of cost.

The last caravan, which left southern Mexico in March, received so much media attention, particularly during its final days, that it appears to have set the groundwork for the current, larger exodus, said many migrants. The current group is exponentially bigger than previous caravans. Hondurans, Guatemalans and Salvadorans who missed their chance this spring decided that this time, they would rush to join the group.

By the time Irma Rosales heard about the caravan in El Salvador, the migrants were already nearing the Guatemala border. Her husband had been murdered a year earlier, she said, and after she reported the crime to the police, the MS-13 threats began naming her.

"I didn't have the money to pay for a coyote, so the caravan was the only way," she said.

After she saw the images of the group on television, she typed "caravana migrante" into Google and saw that the migrants were expected to reach the Guatemala-Mexico border in two days, on October 19. She paid about $10 for three separate bus tickets, travelling for 16 hours, making it to the border on time to catch the caravan.

Then she bought a Mexican phone card and texted her cousin in Chicago.